Before you proceed with moving to Pro Tools (if you do not know exactly why you want Pro Tools, there are better options...):
 
1. delete %APPDATA%\Cakewalk\<Sonar version>\TTSSEQ.INI
 
2- (if still no luck).

AllenKahn
Referring to azslow3 point #1:  I have tried to connect the midi keyboard through the midi in ports of both my audio interfaces (Fast Track Ultra and Tascam US-1800). Do I need to uninstall all instances of these devices?  I do not recognize any device name that corresponds to my connecting the Keystation Pro directly to my laptop.  Any downside to unintentionally uninstalling the wrong device?

In the Windows device manager, select "Show not shown devices" option (I do not remember how it is called exactly in English) and check all your MIDI devices. If you see any "orphaned" (not currently existing) devices, delete them. If you see "-2" ("-3", etc) in names of some existing devices, delete them (then unplug/plug to reinstall, can also happened with MIDI ports of audio interfaces).

1.  Input echo is enabled (remember it worked once, saved project, reopened project and did not work).
2.  Deleted %APPDATA%\Cakewalk\<Sonar version>\TTSSEQ.INI.  No cigar.
3.  Deleted every device for fast track ultra and or midi from audio inputs and outputs,  software devices, sound video and game controllers, and anything I could find remotely applicable (did this several times). No cigar.
 
I also tried this on two different computers.
 
Here is something else interesting - On the off chance that I am getting input from my keyboard, if I save the project and reopen it, it still works, but if I close and reopen Sonar it no longer registers inout from the keyboard.
 
Still open to suggestions...

Does any Synth in the project has "MIDI Output" enabled?
Do you have Drum Maps defined?
 
Also in the OP you write "Sonar Platinum Producer"... There was no such thing. There was "Sonar Platinum" and there was "Sonar Producer" (long time before that).
Have you tried CbB?

The synth I am testing with has midi output enabled.
Not sure about drum maps defined.  I have never used that feature.  I am testing with SI Electric Piano so I am not sure what drum maps have to do with it. 
What I am trying to understand is why it works (sometimes) on a new project until I close and reopen either the project or Sonar.
Pardon my ignorance but I am not sure what CbB is referring to. 
I am using Sonar Platinum, 23.10.0 Build 14.  I have been using Sonar since version 8.  I like it and really don't to switch. Just frustrated because my Keystation was working fine until the update right before Bandlab acquired Cakewalk.
 
 
 
 

I posted this in another thread, and I don't know if this is relevant or not, but I've had to retire my MidiSport 2x2 (also an MAudio based device) as since the 1803 Windows Update, it was constantly disconnecting itself within CbB/SONAR.
 
Windows "claims" it's still connected, but none of the midi messages get to/from any software.
 
It works for a few minutes, then Cakewalk says it's gone away and it's a pain for it to recognise it again. It never did this before I upgraded to 1803.
 
I tried the 2x2 on a different machine, and it seemed to be stable, but it wasn't being thrashed the way it was in the studio. It was used as my Mackie MCU / XT interface, so it was getting all the automation info as well as constant timecode updates.
 
I'm now using a MidiTech MidiFace 8x8 without issues.
 
However, there may be something in the original 1803 update that has destablised M-Audio drivers.
 
Note, I've not tried this again since the Windows 10 Version 1803 Cumulative Update KB4340917 so it might be worth giving this a try to see if it fixes the issue, as it's done for Maschine etc.
 

AllenKahn
The synth I am testing with has midi output enabled.
Not sure about drum maps defined.  I have never used that feature.  I am testing with SI Electric Piano so I am not sure what drum maps have to do with it. 
What I am trying to understand is why it works (sometimes) on a new project until I close and reopen either the project or Sonar.
Pardon my ignorance but I am not sure what CbB is referring to. 
I am using Sonar Platinum, 23.10.0 Build 14.  I have been using Sonar since version 8.  I like it and really don't to switch. Just frustrated because my Keystation was working fine until the update right before Bandlab acquired Cakewalk.

CbB is Cakewalk by Bandlab. It is essentially Sonar Platinum core program, but it is getting updates.
 
Another question: what you enabled in the MIDI devices preferences? I hope nothing except the port to which the keyboard is connected.
Try to disable Synth MIDI output.
 
I know that sounds like make no sense, but all that does... Cakewalk enumerate all MIDI into one array (real inputs, Synth outputs, Drum maps) and when something is "shifted" (stuck/disappear/appear/etc.) they not always reposition things correctly.
Previous CbB update has fixed something in that area (as they claim).
 

I am using Sonar Platinum, 23.10.0 Build 14 also.  MIDI keyboard input is stable here, but I am not currently using M-Audio MIDI drivers.
 
I noticed that the TASCAM US-1800 also has a 5-pin MIDi connection.  Did you try this yet?

Hi folks, this same problem has been driving me crazy for months now! I’ve got an Oxygen 49 and 88. I never seem to have problems with the 88 but the 49 is always intermittent. I always assumed it was a windows problem and have trawled the internet for usb fixes but nothing seems to have worked. I’ve uninstalled everything, devices and software, reinstalled it all again and everything is fine for a short time. Then out of the blue I open up CbB or Platinum and there’s nothing coming from my 49. It’s so frustrating and I’m about to give up on lot. :(

I’m going to try some of the suggestions here and see what happens. It’s good in one way I suppose to know that it’s not just me having this problem. I thought I was going nuts! :)

Cheers

Nifty9993
Hi folks, this same problem has been driving me crazy for months now! I’ve got an Oxygen 49 and 88. I never seem to have problems with the 88 but the 49 is always intermittent...

Nifty, your system info would be helpful... Win 7 or Win 10?  Desktop or laptop?  You say the 88 works fine, but not the 49, so I'd start there. Are they on the same machine? What are the differences?  Is the 49's firmware up to date?

Hi Jon
 
Sorry, I didn't want to hassle anyone so didn't think to post any major details, I was just glad to see it wasn't just me having this problem :)
 
I'm on windows 10, fully updated. I have used the 88 an 49 attached to the computer at the same time and not had problems in the past. I'm not using a usb hub, both are straight into usb ports on the back of the pc, which is an i7. Lately I have just had the 49 left plugged in. 
 
To show how intermittent the problem is, yesterday afternoon the input failed on the 49 after about 30 minutes of playing. Usually I have to go into devices in sonar, uncheck the 49 from the list, restart CbB (or platinum) go straight to devices and recheck the 49 and everything seems ok again. Except yesterday this didn't work at all. No matter how many times I did this CbB just wouldn't get any signal from tge 49. After half an hour trying I gave up and shut the pc down.
 
Last night I went back to it, turned the pc on and opened up CbB. The 49 was still checked in devices after the afternoon antics and it was working perfectly. I played for a couple of hours without a single problem! 
 
I think it's either a windows problem or maybe as simple as a cable problem. Think I'll pick up a new cable and see what happens in the future.
I understood the 49 is plug and play and didn't need any drivers installed to use it.
It's brand new, only had it a couple of months so not sure on firmware. How would I check or update firmware on the keyboard?
 
Thanks for listening anyway :)
Cheers